Abstract

Objective To provide a valid estimate of singleton neonatal mortality based on birthweight and gestational age at delivery.

Design Record linkage of maternity data and neonatal mortality data.

Setting Scotland, UK.

Population All singleton preterm deliveries from 24 to 36 weeks inclusive between 1985 and 1994.

Main outcome measure Neonatal death.

Results There were 625,646 liveborn singleton deliveries over the study period, of which 33,912 were preterm (5.4%). The overall neonatal mortality in the preterm group was 41/1000 and the data have been presented by both gestational age and birthweight. The neonatal mortality rate fell with advancing gestation from 795/1000 live births at 24 weeks to 9/1000 live births at 36 weeks and was higher at the extremes of birthweight for a given gestational age. There was a significant increase in the proportion of babies delivered iatrogenically over the study period (χ test for trend P < 0.001).

Conclusion This is the largest recent series to consider neonatal mortality using both birthweight and gestational age. These figures will be of use in obstetric management when elective preterm delivery is considered, and for providing prognostic guidance following preterm delivery.

INTRODUCTION

Preterm delivery and low birthweight are major causes of both neonatal mortality and long term disability1,2. Reliable knowledge of neonatal mortality rates is useful antenatally not only to provide women in preterm labour with prognostic guidance, but also to guide obstetricians who may be considering iatrogenic delivery of a preterm fetus. Such information is also useful postnatally to neonatologists in considering the appropriate level of medical intervention.

There are several studies illustrating the variation in neonatal mortality with either gestation1,3–12 or birthweight at delivery2,7,13,14. A more useful assessment may be made by considering fetal weight together with gestational age. Such an analysis has previously been performed8,15,16 but the data are now over 20 years old and may no longer be applicable to current practice. As very large numbers of deliveries are required to reliably estimate the neonatal mortality for each gestational age/fetal weight group, we have taken advantage of the routinely collected data in the linked Scottish Maternal and Neonatal Database held by the Information and Statistics Division of the Scottish Health Service. This database contains information on pregnancy, birth history, perinatal mortality and infant mortality on all births in Scotland from 1980 to 1995. The most recent 10 years for which complete data are available were chosen for analysis (1985–1994).

METHODS

Details of maternities in Scotland are routinely collected by the Information and Statistics Division on the SMR2 maternity discharge record. The delivery records are linked using probability matching techniques17 with neonatal birth and death records from the General Registrar Office (Scotland). The completeness of coverage of births on SMR2 over the period 1985–1994 is estimated, by comparison with the General Registrar Office birth records, at 98%. The Information and Statistics Division (Scotland) regularly assesses the accuracy of the data by comparing a national sample of records with the clinical case notes. An assessment of 650 SMR2 records from 1991 showed 99% accuracy for demographic information, 99% accuracy for birthweight and 950 accuracy for estimated gestation.

All singleton preterm (24–36 weeks inclusive) deliveries in Scotland resulting in a live birth over a 10–year period were identified from the linked database. Gestational age was the best estimate from menstrual and ultrasound dates. Birthweight, measured on the day of delivery, was divided into 250 g strata, and the neonatal mortality rate (ie, death of a liveborn baby within the first four weeks of life per 1000 livebirths) was calculated for each gestational age/fetal weight group where at least 10 live births and two neonatal deaths had occurred. These parameters were arbitrarily selected to exclude data at extremes of the fetal weight strata with small numbers, which it was felt would produce unreliable estimates. For completeness in two instances (32 weeks/> 2250 g and 34 weeks/> 3000 g) 10 live births and one neonatal death were selected as the parameters to avoid gaps in central positions.

The incidence of lethal abnormalities over the same period was derived from the Scottish Stillbirth and Neonatal Database. This information comes from a routine annual survey of all stillbirth and neonatal deaths in Scotland performed by the Information and Statistics Division (Scotland) with clinical information provided by obstetricians and paediatricians.

RESULTS

There were 625,646 singleton deliveries over the study period, of which 33,912 (5.4%) were preterm (Table 1). Of these preterm deliveries, 56% were spontaneous vertex deliveries, 8% were instrumental deliveries, and 33% caesarean sections. The remaining 3% were either complex compound presentations or no details were recorded. The mean age of the women was 26 years (SD 5 years). Within the preterm group, there were 1400 neonatal deaths (Table 2) giving an overall neonatal mortality rate of 41/1000 live births (Table 3). Neonatal mortality was greater with low gestational age and birthweight.

Table 1. Live births by birthweight and completed weeks of gestation (1985–1994).

The percentage of all neonatal deaths due to fetal abnormality increased from 2.8% at 24 weeks gestation to 71% at 36 weeks gestation, and averaged 24% over the total gestation range of 24–36 weeks (Table 4).

Of the 33,912 preterm deliveries, 19,956 were the result of spontaneous preterm labour; the remaining 13,956 women were delivered for obstetrical reasons. As 49% of the neonatal deaths in the spontaneously delivered group were between 24–26 weeks, compared with only 14% in the iatrogenically delivered group (Table 5), direct comparison of the overall neonatal mortality rate for the two groups was not appropriate. When each gestational age in weeks was compared separately, however, the neonatal mortality was consistently higher in the iatrogenically delivered group, compared with those in spontaneous labour. The one exception to this was in the 24 week group where the numbers were very small. With adjustment for gestation, the odds ratio for neonatal mortality in the iatrogenic group, compared with the spontaneous group, was 1.58 (95% CI 1.39–1.80). There was a significant increase in the proportion of babies delivered iatrogenically over the study period (χ2 test for trend, P < 0.001).

Table 5. Iatrogenic deliveries by age of gestation (weeks) and year of delivery. Values expressed show numbers of deliveries and a percentage of all singleton livebirths.

Gestational age (weeks

Year of delivery

24

25

26

27

28

29

30

31

32

33

34

35

36

1985

1(6)

5(25)

9(22)

17(33)

27(46)

39(50)

51(52)

56(45)

86(43)

113(45)

171(38)

226(37)

432(33)

1986

1(8)

3(15)

7(18)

19(46)

35(50)

45(49)

48(54)

58(59)

94(47)

103(40)

172(39)

222(36)

420(31)

1987

1(7)

4(17)

8(18)

19(40)

31(45)

44(59)

58(55)

73(57)

112(52)

124(45)

145(37)

232(37)

439(34)

1988

0

0

11(31)

16(42)

33(45)

43(52)

60(54)

80(53)

119(51)

150(53)

178(43)

260(39)

487(35)

1989

0

1(4)

11(30)

16(39)

31(52)

34(49)

56(62)

57(52)

101(52)

118(48)

177(43)

233(35)

488(37)

1990

1(4)

2(6)

11(25)

24(45)

34(50)

43(53)

58(50)

57(48)

103(51)

120(45)

189(41)

271(39)

527(38)

1991

1(4)

5(15)

16(32)

24(43)

31(53)

53(61)

56(57)

86(59)

104(50)

147(53)

193(45)

300(41)

474(37)

1992

4(19)

5(14)

16(36)

33(57)

36(51)

51(63)

64(58)

68(56)

92(52)

121(49)

213(46)

259(39)

525(37)

1993

4(22)

5(16)

12(30)

21(50)

34(54)

51(62)

51(56)

76(57)

102(60)

103(53)

208(46)

251(40)

531(42)

1994

5(21)

8(24)

17(46)

29(59)

44(67)

50(60)

63(68)

95(67)

128(63)

140(54)

226(48)

280(43)

495(41)

DISCUSSION

Table 3 provides a guide to the neonatal mortality for any given preterm weight/gestational age combination and includes standard errors that may used to obtain 95% confidence intervals. At the extremes of weight for each gestational age group these confidence intervals are wide, illustrating the low numbers of deliveries at the extremes and the problems inherent in estimating reliable neonatal statistics. The data may be used to estimate the risks of delivery, predict outcome after delivery or infer the degree of benefit in prolonging pregnancy. Appropriate interpretation of these tables, however, requires accurate establishment of gestation, accurate measurement of the weight, and knowledge of any underlying fetal abnormality which might significantly alter the prognosis.

Confirmation of accurate gestation is most easily obtained in the first or early second trimester by measuring embryonic or fetal crown-rump length or fetal biparietal diameter18. In prospective series of 185 embryos of precisely known gestation (pregnancies resulting from assisted conception techniques) Wisser et al.19 noted that measurement of embryonic length had a mean 95% confidence interval of 4.3 days. It is, however, more difficult to establish gestation later in pregnancy, and great caution should be used in deriving fetal gestation from ultrasound biometry beyond 20 weeks as this is confounded by increasing variation in fetal size.

After delivery, neonatal weight is easily established by accurate weighing of the baby. Estimation of the fetal weight prior to delivery, however, is more difficult and there is increasing evidence of the limitations of ultrasound for this purpose20. Abdominal circumference most clearly correlates with birthweight21. The percentage error in estimation of birthweight with abdominal circumference is lowest at 3.5 kg and increases with both smaller and larger babies, such that the estimated error in birthweight at 200 mm abdominal circumference (average 900 g fetal weight) approaches ± 20%. Caution is therefore required at the earlier gestations where the information is of the greatest clinical importance.

Consideration has been given to the problem of correcting the data for fetal abnormalities diagnosed antenatally. Lethal abnormality data was not available for each preterm weight/gestational age combination and it was therefore not possible to calculate an adjusted neonatal mortality rate excluding these anomalies. Data on neonatal mortality was available from the Scottish Stillbirth and Neonatal Database for each completed week of gestation; in table 4 this is shown as a percentage of the mortality for each given gestation. The contribution of fetal abnormality is small at early gestations (< 10% at 28 weeks or before) but is of much greater importance at later gestations as problems of prematurity play a lesser role. The prognostic significance of any abnormality, therefore, may be assessed on an individual basis.

It has been recognised that a lower than expected birthweight increases the likelihood of adverse neonatal events22, and this is in keeping with the findings here. What is less well recognised is the increased mortality at the high extremes of expected weight for gestation, a finding which might be explained in part by an increased incidence of maternal impaired glucose tolerance, gestational diabetes or pre-existing diabetes mellitus in that group. It is also interesting that the iatrogenically delivered group had a significantly higher neonatal mortality than the spontaneously delivered group, and it is suggested that this may in turn reflect an increased incidence of underlying pathology precipitating the delivery, particularly pregnancy-related hypertensive disease, placental abruption or chorioamnionitis following preterm pre-labour membrane rupture. Further work is required to establish whether these associations exist.

It is possible that these data represent an overestimate of neonatal mortality as antenatally administered steroids, the use of which has increased over recent years, are expected to improve the neonatal outcome23. As routine SMR2 data collection about steroid use only began in 1996, it was not possible to assess their impact. Another cause of over-estimation may be related to improved care: neonatal mortality has fallen by 36% in the 24–36 week gestation groups between 1985 and 1994. As pooled data are required to provide sufficient numbers for meaningful statistical analysis, present day survival is likely to be higher than that averaged over the study period. The increase in iatrogenically delivered babies over the study period may reflect the anticipated improved survival, with a reduction in the threshold for delivery in either fetal or maternal interests.

CONCLUSION

This is the largest recent series to consider neonatal mortality using both birthweight and gestational age. Within the limitations discussed above, these figures will be of use in obstetric management when elective preterm delivery is considered, and for providing prognostic postnatal guidance. There is potential to expand this work by looking for associations between the outcome of the preterm births and a number of obstetric conditions, previous obstetric history and demographic factors. Data could also be prepared for multiple pregnancies and other important subgroups.

Acknowledgements

The authors would like to thank Professor I. A. Greer for helpful suggestions, and Dr P. Owen for critical reading of the manuscript.